This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-068420, filed Mar. 15, 1999; and No. 11-068421, filed Mar. 15, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to an electrode substrate for a reflection type liquid crystal display device and to a reflection type liquid crystal display device, and in particular, to an electrode substrate provided with a light scattering film for a reflection type liquid crystal display device, which enables it to obtain a display plane excellent in brightness and in display quality, and wide in viewing angle, and also to a reflection type liquid crystal display device provided with such an electrode substrate.
The main components of a liquid crystal display device are generally constituted by a pair of electrode substrates each provided with a polarizing film and electrodes for driving a liquid crystal, and the liquid crystal filled in a space between these electrode substrates.
On the occasion of displaying an image in this liquid crystal display device, a voltage is impressed between these facing electrodes so as to cause a change in the state of orientation of the liquid crystal molecules filled between the electrode substrates, thereby controlling not only the plane of polarization of the light passing through the liquid crystal but also the transmittance or non-transmittance of the light through the polarizing film.
As for the type of liquid crystal display device, a transmission type liquid display device of a lamp-built-in type such as so-called back-light or light-guide type is widely employed today. In this liquid crystal display device, a light source (lamp) is arranged on the back or side of an electrode substrate disposed on the back of the device (i.e. an electrode substrate which is disposed on the side remote from a viewer among a pair of electrode substrates between which the aforementioned liquid crystal is filled, which is hereinafter referred to as a back electrode substrate), thereby introducing a light from the back electrode substrate into the display plane, thus enabling it to display a bright image.
In view of the advantageous characteristics that power consumption can be minimized and total weight can be reduced, the application of a liquid crystal display device to a portable display device such as a mobile machine has been increasingly expected now.
In the case of lamp-built-in type liquid display device however, the power consumption by the built-in light source (lamp) is relatively large (for example, the lamp consumes almost the same degree of electric power as that of CRT or of plasma display device). Therefore, the lamp-built-in type liquid display device is disadvantageous in that the serviceable time of battery is short, and that the total weight and size of the device would be increased due to a relatively large volume percentage to be occupied by the battery in relative to the entire device. Namely, it cannot be said that the aforementioned advantages that a liquid display device inherently have are fully utilized by the lamp-built-in type liquid display device.
In view of the aforementioned circumstances, a reflection type liquid crystal display device not provided with a light source (lamp) has been proposed. This reflection type liquid crystal display device is provided on the back electrode substrate thereof with a light reflective plate having a light reflecting function or provided with a reflective electrode which functions not only as a liquid crystal driving electrode but also as a light reflective plate. In this case, an external light such as a room light or natural light is allowed to enter into the liquid crystal display device from the viewer""s side electrode substrate (i.e. an electrode substrate which is disposed on a viewer""s side among a pair of electrode substrates filling a liquid crystal therebetween), and this incident light is then allowed to be reflected by the aforementioned light reflective plate or the reflective electrode, the reflected light being emitted from the electrode substrate disposed on a viewer""s side, thus displaying an image.
As for the back electrode substrate to be employed in this reflection type liquid crystal display device, ones shown in FIGS. 1 and 2 for instance are already known.
The liquid crystal display device shown in FIG. 1 comprises a viewer""s side electrode substrate A, a back electrode substrate B, and a liquid crystal layer 10 interposed therebetween. The viewer""s side electrode substrate A is constructed such that a color filter 16 is formed on a glass substrate 11a, and a transparent electrode 15 is formed on the color filter 16.
The back electrode substrate B shown in FIG. 1 is constructed such that an insulating film 13 having a roughened surface for scattering a light is formed on a glass substrate 11b provided on the surface thereof with a TFT (thin film transistor) array 18, and additionally, a metallic reflective film 12 functioning also as a liquid crystal driving electrode is selectively formed on the insulating film 13 so as to make the location of metallic reflective film 12 coincide with each pixel, the underlying TFT array 18 being subsequently interconnected with the metallic reflective film 12 through via-holes 19.
The liquid crystal display device shown in FIG. 2 comprises a viewer""s side electrode substrate A, a back electrode substrate B, and a liquid crystal layer 20 interposed therebetween. The viewer""s side electrode substrate A is constructed such that a color filter 26 is formed on a glass substrate 21a, and a flattening layer 24 and a transparent electrode 25a are formed on the color filter 26. A polarizing film 27a is formed on the opposite surface of the glass substrate 21a. 
The back electrode substrate B shown in FIG. 2 is constructed such that a transparent electrode 25b is formed on one surface of a glass substrate 21b, and a polarizing film 27b and a metallic reflective film 22 are disposed on the opposite surface of the glass substrate 21b. 
However, the reflection type liquid crystal display device of this kind is accompanied with a problem that, since the aforementioned metallic reflective film is employed for reflecting an incident light, the viewing angle is restricted depending on the position of the external light source.
Moreover, the back electrode substrate B shown in FIG. 1 is also accompanied with problems that the step of forming a roughened surface of the insulating film for securing a sufficient viewing angle as well as the step of forming the via-holes 19 for securing an electric connection between the metallic reflective film 12 and a circuit wiring (the TFT array 18) are complicated, and at the same time, the surface roughness of the metallic reflective film 12 is prominent, thereby raising a problem with respect to the orientation of liquid crystal.
On the other hand, the back electrode substrate B shown in FIG. 2 is also accompanied with problems that since the metallic reflective film 22 is disposed on the back surface of the substrate, the light path of the reflected light is caused to differ from that of the incident light due to the thickness of the substrate 21b, so that, due to this difference in light path, the light that has passed through a pixel and reflected by the metallic reflective film 22 is caused to enter into a neighboring pixel to generate a display defect such as color mixture, or otherwise, the incident light is caused to be reflected not only from the surface of the transparent electrode 25b but also from the metallic reflective film 22 disposed on the back surface of the substrate, thereby generating a dual image.
As means for solving the aforementioned problems, the present inventors have already proposed an idea of providing a light scattering film as set forth in Japanese Patent Unexamined Publications H7-28055 and H7-98446.
According to this proposal, a light scattering film is disposed on one side of the substrate which faces the liquid crystal, i.e. the light scattering film is disposed on the inner surface of the liquid crystal panel, thereby making it possible to minimize the misregistration (parallax) of light in relative to the liquid crystal constituting a switch, thus making it suited for displaying a fine image.
This light scattering film employed in this structure is a coated film wherein transparent particles are dispersed in a transparent resin, the refractive index of the transparent particles being selected to differ from that of the resin. When this light scattering film is disposed in this manner, the light scattering property can be easily secured.
However, it is demanded, for the purpose of effectively scattering the light, to control the diameter of the transparent particles to limit within the range of 0.4 to 1 xcexcm or more. Since particles of such a large diameter is required to be employed, the resultant surface of the light scattering film that has been formed as a coat film is highly roughened as high as 0.2 to 1 xcexcm, thereby interfering the orientation of the liquid crystal. Therefore, an additional flattening film made of a transparent resin having a refractive index which differs from that of the light scattering film is required to be further laminated on the light scattering film.
Moreover, since a coating liquid for forming the light scattering film is formed of a dispersion containing inorganic particles or plastic beads as the transparent particles, the filtering for sorting transparent particles having a proper particle size is difficult to perform. Further, there is much possibility that foreign matters may be mixed into the coating liquid on the occasion of filtering the transparent particles, or foreign matters due to a secondary aggregation may be produced. If a coat film is formed by making use of such a coating liquid, protrusions due to such foreign matters or transparent particles of off-spec size may be generated, thus frequently giving rise to a serious defect such as a display defect in the liquid crystal panel.
Further, a coat film wherein inorganic particles are dispersed is more likely to cause a transmitted light or reflected light to become yellowish, thereby making it difficult to reproduce a snow-white (or so-called paper white) color.
The present inventors have also proposed an idea of generating a light scattering by making use of an optical scattering element such as a micro-lens which is made of a photosensitive resin material. Although it is certainly possible to obtain an excellent light scattering by making use of this technique, it requires a manufacturing process such as a photolithography, thus leading to an increase in manufacturing steps and hence, to an increase in manufacturing cost.
Additionally, it is also required to make the optical scattering element (such as a micro-lens) into a random pattern in order to prevent the generation of a coloration such as rainbow color due to the moire to be generated by a regular pattern. It is required in the photolithography to use a photomask for patterning exposure. However, it is difficult to prepare a photomask of large area and having a random pattern for patterning exposure.
Further, although the micro-lens is required to be formed while controlling its height within the range of 1 to 2 xcexcm, it also invites a problem that it is difficult to flatten this uneven surface.
An object of the present invention is to provide an electrode substrate for a reflection type liquid crystal display device which enables it to obtain a displayed image wide in viewing angle irrespective of the position of an external light source, free from display defects and capable of obtaining a bright image.
Another object of the present invention is to provide a reflection type liquid crystal display device which is provided with such an electrode substrate and is low in manufacturing cost.
Namely, according to this invention, there is provided an electrode substrate for a reflection type liquid crystal display device, which comprises a substrate, and a light scattering film formed on the substrate and including a transparent matrix resin and scattering particles made of a resin, wherein the light scattering film is formed from a coating of a coating liquid dissolved in a solvent the matrix resin and a resin for forming the scattering particles which has a different refractive index from that of the matrix resin the scattering particles are dispersed in the transparent matrix resin as a result of a phase separation due to a low compatibility between these resins as the solvent is evaporated from the coating liquid, and the size and dispersed state of the scattering particles are at least two-dimensionally randomized as viewed from a front of the light scattering film.
According to this invention, there is also provided a reflection type liquid crystal display device comprising a viewer""s side electrode substrate, a back electrode substrate, and a liquid crystal layer interposed between the viewer""s side electrode substrate and the back electrode substrate, wherein the viewer""s side electrode substrate or the back electrode substrate is formed of an electrode substrate comprising a substrate, and a light scattering film formed on the substrate and including a transparent matrix resin and scattering particles made of a resin, the light scattering film is formed from a coating of a coating liquid dissolved in a solvent the matrix resin and a resin for forming the scattering particles which has a different refractive index from that of the matrix resin, the scattering particles are dispersed in the transparent matrix resin as a result of a phase separation due to a low compatibility between these resins as the solvent is evaporated from the coating liquid, and the size and dispersed state of the scattering particles are at least two-dimensionally randomized as viewed from a front of the light scattering film.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.